1. Field of the Invention
The present invention relates to piezoelectric ceramic compositions and piezoelectric ceramic elements using the same, and particularly to a piezoelectric ceramic composition advantageously used for piezoelectric ceramic elements, such as piezoelectric ceramic filters, resonators, and oscillators, and to a piezoelectric ceramic element using the piezoelectric ceramic composition.
2. Description of the Related Art
In general, a piezoelectric ceramic composition used for piezoelectric ceramic elements, such as piezoelectric ceramic filters, oscillators and resonators, has substantially been composed of lead titanate zirconate (Pb(TixZr1-x)O3) or lead titanate (PbTiO3). However, such a piezoelectric ceramic composition contains a large amount of lead whose oxides vaporize during the manufacturing processes, and consequently, degrade the uniformity of the products. In order to prevent the uniformity from being degraded due to the vaporization of lead oxides, it is preferable that the piezoelectric ceramic composition contains substantially no lead.
In contrast, a piezoelectric ceramic composition substantially composed of a bismuth layered compound, such as SrBi2Nb2O9, does not cause the problem described above because it does not contain lead oxides.
However, the materials used in piezoelectric ceramic elements, particularly piezoelectric oscillators, are generally desired to have a high maximum value, Qmax-factor, of the electrical quality factor Q (1/tan xcex4) in the frequency band, that is, between the resonant frequency and the anti-resonant frequency. A piezoelectric ceramic composition substantially composed of a bismuth layered compound does not exhibit a Qmax-factor which is sufficient for practical use.
Accordingly, the inventors of the present invention disclosed a piezoelectric ceramic composition in Japanese Unexamined Patent Application Publication No. 2001-328866. Specifically, this piezoelectric ceramic composition is substantially composed of a bismuth layered compound containing Sr, Bi, Nb and O, and the bismuth layered compound satisfies the relational expressions 0.275xe2x89xa6a/cxe2x89xa60.5 and 4xe2x89xa6(2a+3b)/cxe2x89xa64.5, wherein a, b, and c are the molar fractions of Sr, Bi and Nb, respectively. This composition does not contain lead and has an increased Qmax-factor. Thus, it is useful as a material of piezoelectric ceramic elements exhibiting a Qmax-factor sufficient for practical use.
However, it has been found that Bi vaporizes from the piezoelectric ceramic composition during the manufacturing process, such as a firing process, although the amount of vaporization is smaller than that of the vaporization of lead. This is because the bismuth content in the bismuth layered compound, such as SrBi2Nb2O9, is higher than the value derived from the stoichiometry, that is, the molar fractionsb and c of Bi and Nb of the bismuth layered compound are expressed by b/cxe2x89xa71. As a result, the composition ratio is varied and, thus, the desired characteristics are not obtained, occasionally. An expensive sagger or the like is, therefore, required for the firing process. From the viewpoint of reducing manufacturing costs, it is desired to use a ceramic composition leading to characteristics as close to target characteristics as possible without using an expensive sagger. Preferably, the bismuth content is substantially equal to or less than the stoichiometric value.
Accordingly, an object of the present invention is to provide a piezoelectric ceramic composition mainly containing a bismuth layered compound whose bismuth content is equal to or less than the stoichiometric value and not substantially containing lead and which can be advantageously used for piezoelectric ceramic elements exhibiting a Qmax-factor sufficient for practical use, and to provide a piezoelectric ceramic element using the piezoelectric ceramic composition.
The present invention is directed to a piezoelectric ceramic composition comprising a bismuth layered compound containing Sr, Bi, Nb, and a metal element other than Bi having a valence of 3. The bismuth layered compound satisfies the relational expressions: 0.275 less than a/c less than 0.5; 0.9xe2x89xa6b/cxe2x89xa61; 0 less than x/cxe2x89xa60.175; and 0.5 less than (a+3x/2)/cxe2x89xa60.7, wherein a, b, c and x represent the molar fractions of Sr, Bi, Nb, and the metal element other than Bi having a valence of 3, respectively.
Preferably, the metal element other than Bi having a valence of 3 is at least one selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er and Yb.
The piezoelectric ceramic composition may further comprise less than about 0.25 mol of Ta relative to 1 mol of the Nb contained in the bismuth layered compound.
The piezoelectric ceramic composition may further comprise Mn in an amount equivalent to about 1.5 percent by weight or less calculated as MnCO3 and based on the weight of the bismuth layered compound.
The present invention is also directed to a piezoelectric ceramic element including a piezoelectric ceramic formed of the piezoelectric ceramic composition and an electrode disposed on the piezoelectric ceramic
By setting the composition ratio of the bismuth layered compound so as to satisfy the relational expressions: 0.275 less than a/c less than 0.5; 0.9xe2x89xa6b/cxe2x89xa61; 0 less than x/cxe2x89xa60.175; and 0.5 less than (a+3x/2)/cxe2x89xa60.7, a Qmax-factor sufficient for practical use can be obtained.
Also, by setting the composition ratio of the bismuth layered compound so as to satisfy the relationship b/cxe2x89xa61, variation of the composition ratio resulting from vaporization of Bi in manufacturing processes, such as a firing process, can be prevented, and thus, desired characteristics can be obtained.
When the metal element other than Bi having a valence of 3 is at least one selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er and Yb, the piezoelectric ceramic composition has a remarkable effect.
In particular, when the metal element other than Bi having a valence of 3 is Nd, the piezoelectric ceramic composition has a more remarkable effect.
Also, by adding less than about 0.25 mol of Ta relative to 1 mol of the Nb contained in the bismuth layered compound, the piezoelectric ceramic compound can have still more remarkable effect. However, more than about 0.25 mol of Ta reduces the Qmax-factor in comparison with when Ta is not added, and there is no point in adding an excessive amount of Ta.
Also, by adding Mn in an amount equivalent to about 1.5 percent by weight or less as MnCO3, the piezoelectric ceramic compound can have further remarkable effect. However, more than about 1.5 percent by weight of Mn reduces the Qmax-factor in comparison with when Mn is not added, and there is no point in adding an excessive amount of Mn.
The above and other objects, features, and advantages of the present invention will no doubt become clear and apparent from the following detailed description of preferred embodiments.